Resonator filter cascade

ABSTRACT

The invention relates to a resonator filter cascade on the basis of acoustic surface waves, for which at least two transversely coupled resonator filters, which are composed of several resonator structures disposed adjacent to one another and form a wave guide for acoustic surface waves, and switching transducers are present.  
     It is an object of the invention to change resonator filter cascades of the known type so that the insertion attenuation of the filters is reduced by decreasing the attenuation as a result of cascading, without using a switching coil.  
     This objective is accomplished by a resonator filter cascade, for which the number of resonator structures ( 21; 23; 31; 33 ), the transducers of which are switching transducers ( 211; 231; 311; 331 ), is more than one in all transversely coupled resonator filters ( 2; 3 ) and the switching transducers, belonging to one and the same transversely coupled resonator filters ( 2; 3 ), are not connected with one another. In addition and pursuant to the invention, two switching transducers ( 211; 311  and  231; 331 ) of different transversely coupled resonator filters (2; 3) are each connected with one another over a pair of electrically conducting connections.  
     The invention can be used, for example, in the case of band pass filters with a relative band width of the order of  0.1 % as well as for oscillators.

[0001] The invention relates to the field of electrotechnology and electronics. Objects, for which the use is possible and appropriate, are components based on acoustic surface waves, such as band pass filters with a relative band width of the order of 0.1 percent, as well as oscillators.

[0002] Resonator filter cascades are known, for which at least two transversely coupled resonator filters, which are composed of several resonator structures, which are disposed next to one another, and form a wave guide for acoustic surface waves, are connected with one another so that each of the two transversely coupled resonator filters contains transducers, which consist of collecting electrodes and prongs, are referred to as switching transducers and are connected over electrically conducting connections with a switching transducer of a different transversely coupled resonator filter, those transducers, which are not switching transducers forming the input transducer in the case of one of the transversely coupled resonator filters—optionally as a parallel connection—and, in an analogous manner, the output transducer in the case of the other transversely resonator filter.

[0003] A special embodiment (G. Martin, B. Wall, R. Kunze, M. Weihnacht, Proceedings 1993 IEEE Ultrasonics Symposium, pages 35-39) contains four transducers, two of which are connected in parallel. Admittedly, in FIG. 1 of this paper, a not cascaded, transversely coupled resonator filter is shown; in the text, however, the use of such transversely coupled resonator filters in a filter cascade is explicitly mentioned. If transducers of different individual filters, connected in parallel in this filter cascade, are connected electrically conducting with one another, then this means that in each case two transducers are operated as a parallel connection as switching transducers

[0004] In the case of a further, special embodiment (DE 42 27 362), the resonator filter cascade consists of two transversely coupled resonator filters. The switching transducers of various transversely coupled resonator filters are connected with one another over a pair of electrically conducting connections, called cascade connections. A complex switching resistance is connected between these connections. In the event that this complex switching resistance is a coil, called a switching coil, the cascading attenuation, resulting from the mismatching of the cascade connection, is reduced and, consequently, the insertion attenuation of the resonator filter cascade is decreased.

[0005] This solution has the disadvantage that the use of switching coils results in too high a cost when the resonator filter cascades are used in electronic circuits.

[0006] It is therefore an object of the invention to change resonator filter cascades of the known type so that the insertion attenuation of the filters is reduced by decreasing the attenuation as a result of cascading, without using a switching coil.

[0007] This objective is accomplished by a resonator filter cascade, for which the number of resonator structures, the transducers of which are switching transducers, is more than one in all transversely coupled resonator filters and the switching transducers, belonging to one and the same transversely coupled resonator filters, are not connected with one another. In addition and pursuant to the invention, two switching transducers of different, transversely coupled resonator filters are each connected with one another over a pair of electrically conducting connections.

[0008] Assuming that the switching transducers are identical, the mismatching during cascading varies inversely with the ratio of the actual conductivity G to the susceptance 2πf_(R)C of the switching transducer at a resonance frequency f_(R) (C=capacitance of the switching transducer). In the case of resonator filter cascades with only one switching transducer per transversely coupled resonator filter, the ratio G/2πf₀C can be increased only at one of the resonances, which originate from the first two wave guide modes and form the passband. However, for the second resonance, the ratio G/2πf₀C is decreased, so that, averaged over the passband, the mismatching resulting from cascading is decreased only relatively little.

[0009] In the case of several groups of switching transducers per transversely coupled resonator filter, of which only those switching transducers are connected electrically conducting with one another, which belong to different, transversely coupled resonator filters, it is possible to decrease the mismatching during cascading for both resonances, which form the passband.

[0010] According to an appropriate embodiment of the invention, those resonator structures, which form the input or output of the resonator filter cascade and also those resonator structures, which contain only switching transducers, are constructed identically.

[0011] In this connection, it is particularly appropriate if, in each case, identically constructed switching transducers of different transversely coupled resonator filters are connected with one another over a pair of electrically conducting connections.

[0012] Pursuant to the invention, several switching transducers may be present in at least some resonator structures. These form a group of switching transducers, in which the switching transducers are disposed consecutively in a direction perpendicular to the prongs and have the same aperture. In this connection, it is particularly advantageous if each group of switching transducers has two switching transducers.

[0013] Advisably, the number of switching transducers of each transversely coupled resonator filter can be two.

[0014] According to an appropriate development, each resonator structure contains one switching transducer. Moreover, the number of switching transducers per transversely coupled resonator filter can be two.

[0015] The sum of the apertures of the two groups of switching transducers per transversely coupled resonator filter can be larger than half the sum of the apertures of all transducers of the transversely coupled resonator filter in question, only one aperture value per switching transducer group being taken into consideration in this sum.

[0016] For each transversely coupled resonator filter, one of the groups of switching transducers may be disposed below and the other group of switching transducers above the center line of the reflector strip area/transducer prong area extending perpendicularly to the prongs.

[0017] In the event that each group of switching transducers contains two switching transducers, it is appropriate if the overlapping lengths of the prongs in the groups of switching transducers are selected so that they form a function, which is asymmetrical in relation to the center line of the resonator structures extending parallel to the direction of the prongs.

[0018] The inventive resonator filter cascade can be part of a cascade chain, at least the input or output of this resonator filter cascade being connected with the output of the preceding or the input of the following resonator filter cascade by a pair of electrically conducting connections.

[0019] The number of transversely coupled resonator filters per resonator filter cascade may be two.

[0020] It is particularly advisable when at least one component of all pairs of electrically conducting connections forms a common connection, when these components are at the same potential.

[0021] In the event that the potential of these electrically conducting connections differ from one another, a reactance may be connected between electrically conducting connections, which connect the switching transducers of different transversely coupled resonator filters with one another.

[0022] According to a further appropriate development of the invention, in the event that the resonator structures in question contain more than one transducer, the transducer may be connected in parallel or in series in those resonator structures, the transducers of which form the input or output of the resonator filter cascade.

[0023] The invention is explained in greater detail below by means of an example and an associated drawing.

[0024] On a piezoelectric substrate 1, two identical, transversely coupled resonator filters 2 and 3 are disposed, which are composed in each case of three resonator structures 21, 22 and 23 or 31, 32 and 33, which are disposed adjacent to one another. In these resonator structures, the reflectors 212 and 213, 222 and 223, 232 and 233, 312 and 313, 322 and 323 as well as 332 and 333 in the same sequence enclose plane cavities, in which the transducers 211,221 231,311,321 and 331 are disposed. The transducers 211 and 311, 221 and 321 as well as 231 and 331 are constructed identically in pairs.

[0025] The transducers 211 and 311 as well as 231 and 331, which belong to different transversely coupled resonator filters, are connected with one another over the electrically conducting connections 5 and 4. The second connection between the transducers 211 and 311 or 231 and 331 is produced over connections of the collecting electrodes 2212 and 3212 or 2311 and 3311 to the ground potential. Accordingly, the transducers 211, 231, 311 and 331 are operated as switching transducers.

[0026] The transducer 221 is connected with the input 6 of the resonator filter cascade and therefore is the input transducer of the resonator filter cascade. In an analogous manner, the transducer 321 is connected with the output 7 of the resonator filter cascade and therefore is the output transducer of the resonator filter cascade.

[0027] Because they are at the same potential, the adjacent collecting electrodes of the transducers 211 and 221 as well as 311 and 321 are combined into a common collecting electrode. On the other hand, the adjacent collecting electrodes 2211 and 2311 of the transducers 221 and 231, as well as the adjacent collecting electrodes 3211 and 3311 of the transducers 321 and 331 are separated from one another because they are at different potentials. 

1. A resonator filter cascade, for which at least two transversely coupled resonator filters (2; 3), which are composed of several resonator structures (21; 22; 23 and 31; 32; 33), which are disposed adjacent to one another, and form a wave guide for acoustic surface waves, are connected with one another so that each of the two transversely coupled resonator filters (2; 3) contains transducers, consisting of collecting electrodes and prongs and referred to as switching transducers (211; 231 and 311; 331), the coupling switches (211; 231 and 311; 331) being connected over electrically conducting connections (4; 5) with a switching transducer (311; 331 and 211; 231) of a different, transversely coupled resonator filter and those transducers (221; 321), which are not switching transducers, forming the input transducer at one of the transversely coupled resonator filters (2) and the output transducer at one of the other transversely coupled resonator filters (3), wherein the number of resonator structures (21; 23; 31; 33), the transducers of which are switching transducers (211; 231; 311; 331), is greater than one in all transversely coupled resonator filters (2; 3) and the switching transducers, belonging to one and the same transversely coupled resonator filter (2; 3), are not connected with one another, and that in each case two switching transducers (211; 311 and 231; 331) of different transversely coupled resonator filters (2; 3) are connected with one another over in each case one pair of the electrically conducting connections.
 2. The resonator filter cascade of claim 1, wherein those resonator structures, which form the input and output of the resonator filter cascade, are in each case constructed identically with one another, and also those resonator structures, which contain only switching transducers, are in each case constructed identically with one another.
 3. The resonator filter cascade of claim 2, wherein switching transducers of the same structure (211; 311 and 231; 331) of different transversely coupled resonator filters (2; 3) are connected with one another over a pair of electrically conducting connections (4; 5).
 4. The resonator filter cascade of claim 1, wherein, in at least several resonator structures (21; 22; 23; 31; 32; 33), several switching transducers are present and these switching transducers form a group of switching transducers, in which the switching transducers are disposed consecutively in a direction perpendicular to the prongs and have the same aperture.
 5. The resonator filter cascade of claim 4, wherein each group of switching transducers contains two switching transducers.
 6. The resonator filter cascade of claim 1, wherein each resonator structure (21; 22; 23; 31; 32; 33) contains one switching transducer (211; 311; 231; 331).
 7. The resonator filter cascade of claim 6, wherein the number of switching transducers (211; 231 and 311; 331) of each transversely coupled resonator filter (2; 3) is two.
 8. The resonator filter cascade of claim 2, wherein the sum of the apertures of the two groups of switching transducers (211; 231 and 311; 331) per transversely coupled resonator filter (2; 3) is greater than half the sum of the apertures of all transducers (211; 221; 231 and 311; 321; 331) of the transversely coupled resonator filter (2 or 3) in question, only one aperture value per group of switching transducers being taken into consideration in this sum.
 9. The resonator filter cascade of claim 8, wherein, for each transversely coupled resonator filter (2; 3), one of the groups of switching transducers (231 and 331) is disposed below and the other group of switching transducers (211 and 311) is disposed above the center line of the reflector strip area/transducer prong area extending perpendicularly to the prongs.
 10. The resonator filter cascade of claim 5, wherein the overlapping lengths of the prongs in the groups of switching transducers are selected so that they form a function, which is asymmetric in relation to the center line of the resonator structures, extending parallel to the direction of the prongs.
 11. The resonator filter cascade of claim 1, wherein such a resonator filter cascade is part of a cascade chain, at least the input (6) or output (7) of this resonator filter cascade being connected by a pair of electrically conducting connections with the output of the preceding or the input of the following resonator filter cascade.
 12. The resonator filter cascade of claim 1, wherein the number of transversely coupled resonator filters per resonator filter cascade is two.
 13. The resonator filter cascade of claim 1, wherein a component of all pairs of electrically conducting connections forms a common connections, when these components are at the same potential.
 14. The resonator filter cascade of claim 7, wherein, in the event that the potential of these two electrically conducting connections (4; 5) are different from one another, a reactance is connected between two electrically conducting connections (4; 5), which connect the switching transducers (211; 311 and 231; 331) of different, transversely coupled resonator filters (2; 3) with one another.
 15. The resonator filter cascade of claim 1, wherein, in the event that the resonator structures (22; 32) in question contain more than one transducer, the transducers are connected in parallel or in series in those resonator structures (22; 32), the transducers of which (221; 321) form the input (6) or output (7) of the resonator filter cascade. 